Nucleotides & Nucleic Acids (OCR A Level Biology)

Meselson and Stahl were two scientists who investigated DNA replication during the 1950s. 

They grew E. coli bacteria in dishes containing culture media. 

Some bacteria were grown in a culture containing an isotope of nitrogen with a higher molecular weight (15N) than normal nitrogen (14N). 

The bacteria started incorporating the nitrogen isotopes from the culture media into their DNA. As the bacterial colonies grew, the DNA of bacteria grown in a 15N culture became denser than bacteria grown in a normal (14N) culture.

Some bacteria were first grown in the 15N culture and then transferred to the normal (14N) culture.

After letting the bacteria grow for several generations, the scientists extracted and centrifuged the DNA. During centrifugation, the heavier, denser molecules settled towards the bottom of the tube. Figure 2.1 shows some of their results.

Key:

A Bacteria grown in a ¹⁵N culture 

B Bacteria first grown in a ¹⁵N culture and then transferred to the normal (¹⁴N) culture

C Bacteria grown in a ¹⁴N culture

Figure 2.1

Some bacteria were first grown in the 15N culture and then transferred to the normal (14N) culture (tube B). They were left there with enough time to replicate once before the DNA was extracted. 

Suggest where a band would appear on this tube (B) in comparison to tubes A and C. 

Justify your answer to part (a). 

Explain how Meselson and Stahl’s experiment demonstrated the semi-conservative replication of DNA, concerning the results shown in Figure 2.1.

Figure 3.1 shows the hydrolysis reaction of ATP to release energy.

Figure 3.1

The amount of energy released from the hydrolysis of 1 mole of ATP in a living cell is 57 kJ.

Explain why ATP is described as the “universal energy currency”. 

Calculate the amount of free energy in kJ/mol released from the hydrolysis of 0.7 moles of ATP in a living cell. Give your answer to 1 decimal place. 

Compare and contrast the structures and functions of ATP and ADP. 

A DNA sequence coding for a specific protein was analysed. The base composition of the template strand is given in the table below.

Deduce the number of bases in the complementary mRNA strand. 

Calculate the percentage base composition of thymine in the DNA. Give your answer to 1 decimal place. 

mRNA provides a template for protein synthesis. Protein synthesis requires tRNA molecules.

Explain how tRNA is used in the translation stage of protein synthesis. 

State the first tRNA molecule that is used in protein synthesis.

Transcription is the first step of protein synthesis in cells.

Discuss the similarities and differences of the processes of transcription and DNA replication. 

Developing organisms need to control which genes are expressed and transcribed into proteins.

Define the term “homeobox gene”.

Explain how homeobox genes control the development of body plans.

DNA can be obtained from a variety of plant and animal cells.

A group of students tried to purify some DNA from leek cells using the following method. They decided that exact volumes were not necessary.

State the purpose of step 1.

[1]

(ii)

Suggest why a protease enzyme added in step 3 is needed to purify DNA.

[1]

(iii)

The students considered using pineapple juice as a source of protease enzyme.

Suggest why this would not be an appropriate source of protease when attempting to produce a pure sample of leek DNA.

[1]

(iv)

State one important step that the students had left out of their method.

[1]

(v)

Name the process described in step 6.

[1]

Fig.1 shows the structure of a small section of DNA in a cell during transcription.

Fig. 1

Identify the bases Q, R and S as indicated in Fig.1. Explain how you reached your answer.

[2]

ATP is the energy-carrying molecule which fuels chemical reactions within the cell.

Compare the structure of ATP with the structure of a DNA nucleotide.

Scientists analysed the nucleotide base sequence of a DNA molecule extracted from a human cheek cell. It was found that 29% of the nucleotides in this DNA molecule contained the nitrogenous base adenine.

Calculate the percentage of nucleotides that would contain the nitrogenous base cytosine.

Show your working.

The process of transcription is illustrated in Fig. 1.

Fig. 1

Identify molecule X in Fig. 1 and describe its role during transcription.

[3]

(ii)

Identify strand Y in Fig. 1 and state the significance of strand Y during transcription.

[2]

The table below contains a variety of statements that apply to different biological molecules.

Identify which of the statements apply to DNA, messenger RNA and proteins by placing a tick (✓) in the box.

Describe the role of covalent bonds in linking nucleotides together to form a polynucleotide, such as DNA.

DNA replication is described as being 'semi-conservative'.

Explain what this means.

Explain how the structure of DNA facilitates semi-conservative replication of the molecule.

The new DNA molecules that form during semi-conservative replication are identical to each other.

Discuss the importance of this statement.

Describe the role of the ribosome in translation.

Fig. 1 shows the base sequence of a gene that codes for a short polypeptide, the RNA codons and the primary structure of this polypeptide.

Fig. 1

The genetic code is universal across most forms of life.

Explain the importance of this statement.